The present invention relates to a drive control method for a photosensor array including a plurality of photosensors arranged in a two-dimensional direction.
In recent years, pickup devices such as an electronic still camera and a video camera have been prominently propagated. In such a pickup device, a solid state pickup device such as a CCD (Charge Coupled Device) is used as a photovoltaic device for converting the subject image into image signals. As known to the art, a CCD is constructed photosensors (light receiving elements) such as photodiodes and TFTs (Thin Film Transistors) that are arranged to form a matrix. The amount of electron-hole pairs (amount of charge) generated in accordance with the amount of light illuminating the light receiving portion of each photosensor is detected by a horizontal scanning circuit and a vertical scanning circuit so as to detect the brightness of the illuminating light.
In a photosensor system using such a CCD, it was necessary to arrange separately a selecting transistor for putting a scanned photosensor in a selected state. On the other hand, the present inventors have previously developed a photosensor (double gate type photosensor) constructed by a thin film transistor having a so-called “double gate” structure, in which the photosensor itself is enabled to perform the photosensing function and the selecting function.
FIG. 7A is a cross sectional view showing the construction of such a double gate type photosensor 10. As shown in the figure, the double gate type photosensor 10 comprises a semiconductor thin film 11 made of, for example, an amorphous silicon, n+-type silicon layers 17, 18 formed on both edge portions of the semiconductor thin film 11, a source electrode 12 and a drain electrode 13 formed on the n+-type silicon layers 17 and 18, respectively, a top gate electrode 21 formed above the semiconductor thin film 11 with a block insulating film 14 and an upper gate insulating film 15 interposed therebetween, a protective insulating film 20 formed on the top electrode 21, and a bottom gate electrode 22 formed below the semi-conductor thin film 11 with a lower gate insulating film 16 interposed therebetween. The bottom gate electrode 22 is formed on a transparent insulating substrate 19 such as a glass substrate.
In other words, the double gate type photosensor 10 comprises an upper MOS transistor including the semiconductor thin film 11, the source electrode 12, the drain electrode 13, and the top gate electrode 21, and a lower MOS transistor having the semiconductor thin film 11, the source electrode 12, the drain electrode 13, and the bottom gate electrode 22. As apparent from the equivalent circuit diagram as shown in FIG. 7B, it is reasonable to understand that two MOS transistors having the semiconductor thin film 11 as a common channel region, a TG (top gate electrode), a BG (bottom gate electrode), an S (source terminal) and a D (drain terminal) are combined to form the double gate type photosensor 10.
Each of the protective insulating film 20, the top gate electrode 21, the upper gate insulating film 15, the block insulating film 14 and the lower gate insulating film 16 is formed of a material having a high transmittance of a visible light exciting the semiconductor layer 11. The light incident from the top gate electrode 21 is transmitted through the top gate electrode 21, the upper gate insulating film 15, and the block insulating film 14 so as to be incident on the semiconductor thin film 11, with the result that charges (holes) are generated and accumulated in the channel region.
FIG. 8 schematically shows the construction of the photosensor system including the double gate type photosensors 10 in a two dimensional direction. As shown in the figure, the photosensor system comprises a sensor array 100 formed by arranging a large number of double gate type photosensors 10 to form a matrix consisting of an n-number of rows and an m-number of columns, a top gate line 101 and a bottom gate line 102 consisting, respectively, of the top gates TG and the bottom gates BT of the double gate type photosensors 10 that are connected to each other in the row direction of the matrix, a top gate driver 111 and a bottom gate driver 112 connected, respectively, to the top gate line 101 and the bottom gate line 102, a data line 103 consisting of the drain terminals D of the double gate type photosensors 10 that are connected to each other in the column direction of the matrix, and a column switch 113 connected to the data lines 103.
Symbols Vtg and Vbg shown in this figure represent the reference voltages for generating a reset pulse φTi and a read pulse φBi, respectively, which are described hereinlater, and a symbol φpg represents a pre-charge pulse for controlling the timing for applying a pre-charge voltage Vpg.
In the construction described above, the photosensing function is performed by applying a predetermined voltage from the top gate driver 111 to the top gate terminal TG, and the reading function is performed by applying a predetermined voltage from the bottom gate driver 112 to the bottom gate terminal BG so as to supply the output voltage of the photosensor 10 to the column switch 113 through the data line 103 and, thus, to produce a serial data Vout as the output signal.
FIGS. 9A to 9F are timing charts showing the drive control method of the photosensor system. In the first step, a reset pulse φTi shown in FIG. 9A is applied to the top gate line 101 in an i-th column during the detecting operation period (processing cycle in the i-th column) in the i-th row so as to perform a reset operation for releasing the charges accumulated in the double gate type photosensor 10 in the i-th row during the reset period Treset.
After completion of the reset period Treset, a charge accumulating period Ta is started by the charge accumulating function in the channel region. During the charge accumulating period Ta, charges (holes) are accumulated in the channel region in accordance with the amount of light incident from the side of the top gate electrode.
A pre-charge period Tprch, in which the pre-charge pulse φpg shown in FIG. 9E, which has a pre-charge voltage Vpg, is applied to the data line 103 so as to permit the drain electrode to retain the charge, is provided in parallel to the charge accumulating period Ta. After the pre-charge period Tprch, a read pulse φBi shown in FIG. 9C is applied to the bottom gate line 102 so as to turn on the double gate type photosensor 10, thereby starting a read period Tread.
During the read period Tread, the charges accumulated in the channel region serve to moderate the voltage (low level) applied to the top gate terminal TG of the opposite polarity. As a result, an n-channel is formed by the voltage Vbg of the bottom gate terminal BG, and the voltage VD of the data line 103 tends to be gradually lowered with time from the pre-charge voltage Vpg in accordance with the drain current. In other words, the tendency in the change of the voltage VD of the data line 103 depends on the charge accumulating period Ta and the amount of the received light. To be more specific, the voltage VD tends to be lowered moderately in the case where the incident light is dark and the light amount is small so as to decrease the amount of the accumulated charge. On the other hand, the voltage VD tends to be lowered rapidly in the case where the incident light is bright and the light amount is large so as to increase the amount of the accumulated charges. It follows that the amount of the illuminating light is calculated by detecting the voltage VD of the data line 103 a predetermined time after the starting of the read period Tread or by detecting the time required for reaching the particular voltage based on a predetermined threshold voltage.
In the detecting operation period in the succeeding i+1st row (i+1st row processing cycle), the reset pulse φTi+1 shown in FIG. 9B and the read pulse φBi+1 shown in FIG. 9D are applied as in the operation for the i-th row for performing the reading operation. Such an operation is performed for each row of the sensor array 100.
The operation described above covers the case where a double gate type photosensor is used as the photosensor. However, the photosensor system using photodiodes or phototransistors also has the operating steps of reset operation→pre-charge operation→read operation and, thus, follows the similar drive procedures.
However, the conventional photosensor system described above gives rise to problems as pointed out below:
(1) Where the image of a subject is read by using a photosensor array having a plurality of photosensors arranged to form a matrix in two dimensional directions, it was customary to employ a drive control method that a series of processing procedures are performed such that reset pulses and pre-charge pulses are applied to the photosensors for every row of the matrix, followed by applying read pulses the charge accumulating period Ta later, and that the particular procedure is repeated for every row.
As a result, when it comes to a two dimensional matrix having an n-number of rows, the similar operations must be repeatedly performed n-number of times starting with the first row and ending in the last n-th row in order to perform the scanning operation over the entire region of a single screen. In other words, the processing time (scanning time) over the entire region of a single screen is increased with increase in the number of rows of the two dimensional sensor array. As a result, a restriction is generated that the subject must be kept stationary until completion of the scanning operation over the entire region of the single screen. It follows that the practical use of the photosensor array is very much limited.
(2) In the photosensor system using the photosensor of the type that the charges generated by the incident light are accumulated during the charge accumulating period like the double gate type photosensor described above, the charge accumulating period must be set long for obtaining a sufficient detection sensitivity in the case where the subject is dark and, thus, the charges are accumulated in a small amount. On the other hand, the charge accumulating period must be set short to prevent the charges from being saturated in the case where the subject is bright and, thus, the charges are accumulated in a large amount. In other words, in order to read an image of the subject with a suitable sensitivity, it is necessary to set appropriately the sensitivity of the photosensor in accordance with the brightness of the subject. Therefore, where the site in which the photosensor system is used, and the subject itself are changed in various fashions, the brightness of the subject is changed in various fashions depending on the environmental conditions and the kind of the subject. Under the circumstances, it is necessary to perform a trial read operation (or preparatory read operation) immediately before the normal read operation of the subject image so as to obtain a suitable sensitivity. Where the preparatory read operation is performed by the conventional drive control method, the entire screen is read by setting the sensitivity at a suitable value and, if the result of detection is inappropriate, the entire screen is read again by changing the sensitivity. The particular operation is repeated a plurality of times so as to find a set value of sensitivity that permits obtaining an appropriate result of detection. Naturally, the preparatory read operation takes a very long time, giving rise to the problem that it is impossible to start the read operation of the image of the subject promptly with an appropriate sensitivity.